“I’ve been doing some research,” Lily says as we cross the bridge at the bottom of the street. Beneath the parapet, the railway tracks are empty, the hourly train that runs into town every Saturday slowly disappearing into the distance. The sky above is a perfect blue, the electric cables strung along the railway line almost glinting in the sunlight. “According to the internet, I can’t get the tattoo removed on the NHS, not unless a doctor says it’s making me depressed, so that means I’ve got to go private.”

I hurry to keep up with my sister, Lily striding ahead almost as quickly as she’s talking.

“I’ve looked on a couple of different websites and it costs as much as five hundred pounds to get a tattoo like mine removed. I’ve only got a hundred and fifty quid in my bank account. And even if I’m able to save up enough money, apparently it can take months for the laser treatments to totally get rid of the tattoo.”

She glances down at the plaster that’s stuck to the underside of her wrist.

“I wish I could just peel it off like a fake tattoo,” she says bitterly.

As we reach the top of the bridge I can see the back of our house. The budget gazebo is now fully erected in the back garden as it backs on to the railway line. And on the other side of the tracks the parade of shops leads up Cheswick Hill, the bright sunshine making me squint as we start to descend the bridge.

“Maybe I can help,” I say. “Nanna Pegg said she was going to send me some money for my birthday. You can have that when it comes. And if we both help Dad to test his new videogame, that’d be a way to get some extra cash.”

Lily looks at me, her eyes shining in the bright sunlight.

“You’d do that for me?”

I nod my head.

“Of course I would,” I tell her. “You’re my sister.”

As we walk, Lily reaches out to squeeze my hand.

“Thanks, Maisie.”

Our shadows lead the way as we turn the corner at the bottom of the bridge. The railings that fence off the railway tracks are bent and broken in places, the metal posts forced apart by the teenage boys who hang out on the parade at night, riding around on their bikes.

I watch them from my window sometimes when I’m supposed to have gone to sleep. With my bedroom light off, I can see them illuminated in the street lights as they squeeze through the gap, playing dare as they leave stuff lying on the railway tracks, waiting for the next passing train to squash these things flat.

If the railway line wasn’t here, we could get to the shops in just a couple of minutes by sneaking through the gap in our back fence.

“I don’t know why Mum wouldn’t let me come to the shops on my own,” I complain as we walk past the pharmacy that marks the start of the parade. “I mean, I’m ten now.”

Lily laughs.

“Mum wouldn’t let me go to the shop on my own until I’d started secondary school. And then as soon as she did I was the one she kept sending on errands to buy milk and stuff whenever we ran out. You should count yourself lucky.”

Outside the shops, there’re a couple of boys hanging around on their bikes and, as we pass, the taller one who’s wearing a Man City top wolf-whistles at Lily.

“All right, gorgeous,” he says, wheeling his bike round to ride alongside us. “Give us a smile.”

Lily just ignores him.

With his face tightening into a scowl, the boy pulls his bike round in a tight curve in front of us, forcing us to stop to avoid walking straight into him.

“I asked you to smile,” the boy says, an angry tone now in his voice. “Didn’t you hear?”

I glance up at my sister, feeling suddenly frightened, although I’m not exactly sure why.

For a second, Lily stands her ground, her eyes narrowing as she glares back at the boy. Then she pulls her face into a smile, so fake it hurts, but the boy must think this is real enough to count as he wheels off back to his friend.

Grabbing hold of my hand, Lily hurries me on as the boy on the bike mutters something that I don’t understand.

Lily blushes flame-red.

“What did he mean?” I ask as the two boys behind us laugh.

“Don’t worry,” Lily says with a shake of her head. “You wouldn’t understand.”

There’s so much I don’t understand. Sometimes the way the universe works seems simple in comparison with people.

Reaching Kumar’s Convenience Store, Lily pushes the door open, the ring of the bell above it announcing our arrival as we head inside.

“All right, girls?” Mrs Kumar says, looking up from the box of drinks she’s stacking in the glass-fronted fridge. Her bright-red sari is pinned to her shoulder and as she gets to her feet she brushes her hands down the front of it. “What can I get you today?”

“We need some paper plates,” Lily replies, glancing up at the magazines on the racks. “Have you got any?”

Nodding, Mrs Kumar points towards the back of the shop.

“Second shelf down on the right,” she says. “Just past the bin bags.”

I follow Lily as she heads down the aisle, ignoring the display of chocolate bars even though I’m starting to feel a bit hungry again. I remember the mountains of food on the kitchen table. There’ll be plenty to eat when my party gets started.

Picking up a pack of paper plates, Lily turns back to the front of the shop.

“Wait a second,” I say, pointing to the soft-drink bottles on the shelf below. “Didn’t Mum want us to get some drinks too?”

Lily shakes her head.

“Mum’s got a ton of drinks in the fridge. She was just saying that to stop you moaning about me coming to the shop with you.”

I can’t believe it. Mum lied to me on my birthday.

“Don’t stress,” Lily says, noticing the frown on my face. She pulls the twenty-pound note out of her purse. “This means I can keep the change and put it towards the laser treatment I need to get the tattoo removed.”

I should feel happy for Lily, but I can’t help feeling kind of annoyed too as she turns towards the counter. I wish everyone would stop treating me like a little kid.

As Lily places the paper plates on the counter, Mrs Kumar picks them up and scans them through the till.

“Do you want a plastic bag?” she asks.

“Yes, please.”

Pulling one out from under the counter, Mrs Kumar slides the paper plates into it.

“That’ll be two pounds fifty,” she says, handing the bag to Lily. “Are you having a picnic with those paper plates? You’ve certainly got the weather for it today.”

Lily shakes her head.

“It’s Maisie’s birthday. We’re having a party.”

The ever-present smile on Mrs Kumar’s face grows even wider.

“Happy birthday!” she says.

I just blush in reply. I wish Lily wouldn’t embarrass me like this.

On a stand next to the counter, there’s a display of helium balloons – red, blue, pink, purple and gold – each one bobbing gently from its colour-coded ribbon in the breeze from the air-conditioning unit overhead.

Mrs Kumar reaches over the counter and pulls one of the balloons free.

“A birthday balloon for the birthday girl,” she beams, presenting it to me.

I can feel the blush on my face bloom to match the colour of the balloon. If Mum and Dad getting me a birthday badge wasn’t bad enough, now Mrs Kumar is giving me this little kid’s toy. For a second, I think about saying no thanks, but then I realise that the helium inside the balloon might come in handy for one of my experiments. And at least it’s not a pink one.

“Thanks, Mrs Kumar,” I say, looping the ribbon around my wrist. “And could I have a gold one too?” I add cheekily, an idea about how I could use two balloons starting to take shape in my brain.

Mrs Kumar nods her head. “Of course you can. Help yourself, Maisie.”

As I loop the second balloon around my wrist, Lily just grins.

“Come on, birthday girl.”

With the red and gold balloons bobbing behind us, we walk back up the parade. At first I’m a bit worried that we’ll see those boys again, but as I look outside the pharmacy their bikes are long gone. Breathing a sigh of relief, I glance up at Lily.

The sunlight bathes her skin in a golden sheen and, as her long dark hair blows in the breeze, she almost looks like a grown-up.

“What do you want to be when you grow up?” I ask her.

Lily looks surprised.

“I dunno,” she says, shaking her head. “Maybe work in fashion or something. What about you?”

There’s so much I want to do. I’d like to work at the Large Hadron Collider and discover a new elementary particle. I want to solve the Riemann hypothesis – the most difficult problem in maths – and win a million pounds. I might even set up my own tech company to research artificial intelligence. But Lily would think these dreams of mine just prove that I’m a freak.

“Maybe be a videogame designer like Dad,” I say finally. “Or win a gold medal in the hundred metres at the Olympic Games.”

Me and Lily always used to have running races in the back garden. She never let me win, but I reckon I could beat her now that I’m ten.

Lily grins.

“Maybe you’d get a silver medal,” she says. “But I’d definitely get gold.” She wraps the handles of the plastic bag round her other wrist, holding it more tightly in her hand. “I’ll race you home. First one inside the gazebo wins the Olympics. On your marks, get set, GO!”

Lily starts running, her sudden acceleration taking me by surprise. With a squeal of protest, I start chasing after her, the helium balloons dancing behind me in the breeze. But Lily is racing ahead, her long legs easily outpacing mine as she reaches the bottom of the bridge.

“See you later, slowcoach,” she calls over her shoulder.

I shake my head as Lily disappears around the corner. I’ve got to win this race.